Dollar Watch: Timer - LCD Display

Background

It would be handy for
many PIC micro controller circuits to have a small LCD display that
would have a very low power requirement. Trying to find these
displays has been a very difficult search. But I found these
watchs at a local Dollar Store for, you guessed it, One Dollar each.

Operation

Documented

The display defaults to displaying the hour and minute with the colon
blinking (by chance the camera took the photo above when it was off).

Pressing B2 one time causes just the month to be displayed. It can be incremented by pressing B1.
Pressing B2 a second time causes just the Day of the Month to be displayed. It can be incremented by pressing B1.
Pressing B2 a third time causes just the hour and either A or P to be displayed. It can be incremented by pressing B1.
Pressing B2 a forth time causes just the minute to be displayed. It can be incremented by pressing B1.
Pressing B2 a fifth time causes just the seconds to be displayed. It can be incremented by pressing B1.

Undocumented

There is no provision on the watch case to operate Button 3. When
the display is hh:mm and B3 is pressed once the display alternates
between:
hh:mm and mm dd. i.e. between the time and date.
Repeated presses of B3 then act like repeated pressing of B2 to allow
setting.

The movement has notches that would support buttons B4, B5 and B6 but
they have not been installed on this cheap movement. The case has
what appear to be buttons B5 and B6 but these are just for looks i.e.
they are non functional.

Battery

The watch is powered by a single LR41 a 1.5 Volt button cell about 8 mm diameter.

Power Up & Down

If the battery is removed and then installed the watch starts running at 12:00 and Dec. first (12 1).
When the battery is removed the LCD goes blank.

Stopping Clock - Not

Pressing and holding the mode button changes the display from hh:mm to
alternating hh:mm and mm dd but does not stop the clock.

Mods

Timer

To use the watch movement as a timer the battery can be removed and the
battery terminals connected to a pair of wires that will feed the watch
1.5 volts to start the timer. The problem is how to stop the
timer. For example if the crystal is shorted out the LCD goes
blank.
When the crystal is shorted the LCD goes blank, BUT when the short is
removed the LCD resumes the prior count. This means the cheap
watch CAN be used as a timer good for up to one year.

For a timer that will be good for up to 24 hours, many days if you
manually keep track of the days, you can use an analog quartz
watch. I found one for $8 at the local Big Lots store.
That's eight times more than the price of the digital watch that
included time and date functions. If that was built into a
product there's a large impact on the selling price.

LCD Display

The LCD might be removed from the rest of the movement. More needs to be done to understand how to drive the LCD.

There are 13 contacts on the PCB which connect to the glass using a zebra strip.
Contacts No. 1 and No. 13 both have a signal that has three levels:
+150 mv, 0, -150 mv. The time between transitions is about 100 uS.
All the other contacts (No. 2 through No. 12) have signals that switch between +150 mv and -150 mv.

The LCD glass is specifically made to be a watch 12 hour time
display. This means the tens of hours digit can only be "1" or
blank. Notice that the "1" in the time display above
is right aginst the left edge of the glass, i.e. there is not room for
more segments like would be needed to display a "2". In a similar way
the tens of minutes digit can only be blank, 1, 2, 3, 4 or 5.
This reduces the number of segments that need to be controlled to 22
which matches the use of two control and 11 segment contacts.